1. Field of the Invention
The present invention relates to electrolytic cells and particularly to the electrolysis of brine utilizing a cation-permselective membrane.
2. Description of the Prior Art
It is known to obtain increased current efficiency in a process for the electrolysis of brine wherein cation-permselective membranes are utilized to separate an anode and a cathode in a three-compartment electrolysis cell by reducing the concentration gradient on the cell membrane as disclosed in U.S. Pat. No. 3,220,941 wherein current efficiency of such a cell is improved by utilizing sodium carbonate between the two membranes of said cell. Improved current efficiency in a diaphragm cell has also been disclosed in U.S. Pat. No. 3,932,261 by means of the use of electrodes composed of supported foraminous metal sheets.
It is also known from U.S. Pat. No. 3,773,634 that in cation-permselective membrane electrolysis cells for the production of chlorine and sodium hydroxide that maximum current efficiency is obtained by operating the cell at a critical sodium hydroxide concentration of 31-43%.
Various gas-directing electrodes are known for use in electrolytic cells, for instance, reduced operating voltage is obtained according to U.S. Pat. No. 3,168,458 where perforated electrodes are utilized which allow for the transfer of liquid from one side of the electrode to the other. High current efficiency is obtained, according to the teaching of U.S. Pat. No. 3,598,715, in an electrolytic cell for the production of sodium chlorate having an expanded metal cathode in which the gas evolved thereon is directed away from the inter-electrode gap.
There is disclosed in German Offen. No. 2,419,204, an increase in efficiency of the electrodes in a diaphragm cell for the electrolysis of brine is obtained where inclined plates are positioned at the electrodes functioning to guide the gas evolved thereon toward the middle of the electrode chamber for release. A similar design is disclosed in British Pat. No. 1,460,357 and U.S. Pat. No. 3,930,151. In U.S. Pat. No. 3,930,981, a diaphragm cell for the electrolysis of brine is disclosed having perforated metal anodes and baffles to direct anode gases away from the inter-electrodic gap in order to protect the diaphragm against erosion.
The effect of gas evolution at the electrodes on the current overpotential relation and current distribution in diaphragm type electrolytic cells has recently been discussed in the Journal of Applied Electrochemistry, Vol. 6,(1976) pages 171-181. The polarization of a permeable membrane surface in a multicell electrodialysis apparatus is disclosed in U.S. Pat. No. 2,948,668 wherein alternating anion-permeable and cation-permeable membranes are separated by corrugated perforated spaces so as to cause strong turbulence in the flow of liquid through the cell in order to overcome said polarization.
In no one of the above-discussed references is there recognition of the fact that, in a cell for the electrolysis of brine utilizing a cation-permselective membrane, there is a hydroxide ion concentration gradient on the catholyte side of the membrane. At the membrane surface there is a substantially higher concentration of hydroxide ions than in the remaining bulk of the catholyte. The higher concentration of the hydroxide ions on said membrane leads to a lower current efficiency than would otherwise be obtained. The electrolysis method and electrolysis cell apparatus disclosed herein is effective in reducing this concentration gradient and thus increasing the current efficiency of said cell as compared to those of the prior art.
While it is known from U.S. Pat. No. 3,616,444 that so called "gas blinding" of the electrodes in an electrolytic cell for the production of sodium chlorate results in an increased electrical resistance between the anode and cathode of said cell, it is unexpected that the induced turbulent flow between the catholyte and the cell membrane of the electrolytic cell disclosed herein results in increased current efficiency.
It is thus seen that the prior art teaching in the field of the electrolysis of brine to produce chlorine and sodium hydroxide has failed to recognize both (1) the cation-permselective membrane concentration polarization phenomena (wherein hydroxyl ions are present in excess on the catholyte side of the membrane as compared with hydroxyl ions present in the bulk of the catholyte) and (2) the beneficial effect on cell current efficiency obtained by inducing turbulence in the catholyte near the surface of the cell membrane preferably by either directing the evolved cathodic gases away from or toward the permselective membrane of said cell.